Our overall interest is in advancing the understanding of calcium homeostasis in vertebrates. In the current proposal, one of the principal control systems involved in calcium homeostasis, the renal vitamin D endocrine system, will receive the major consideration. A comparative approach will be used involving both mammalian and submammalian vertebrates, represented mainly by the rats, the Japanese quail, and poikilotherms such as amphibia and fish. The major thrust will be concerned with the mechanisms, both physiological and pharmacological, by which the renal vitamin D endocrine system is regulated in avian and mammalian species. Initially, the studies will focus on developing our original observation that estrogens influence the vitamin D endocrine system in Japanese quail. Nevertheless, other endocrine and nutritional factors (PTH; prolactin; and calcium in the dietary and aqueous environment) will also receive attention. In a separate thrust studies concerned with the structure-activity relationships of PTH(1-34), especially the effect of mild oxidation of the molecule with respect to all the known major responses to PTH, will be continued.